Prior art hot runner nozzles often have an uneven distribution of heat along the length thereof when operating in an injection molding apparatus. This uneven distribution of heat causes the temperature of the melt flowing through the nozzle to vary as it travels toward a mold cavity. Any variation in melt temperature can adversely affect the quality of the molded products and is therefore undesirable. The uneven heat distribution of the nozzle results from more heat being lost at the ends of the nozzle than at the midsection of the nozzle. FIGS. 1 and 2 illustrate this pattern of heat loss.
A prior art injection molding apparatus 100 is generally shown in FIG. 1. The injection molding apparatus includes a nozzle 102 that is mounted between a manifold 104 and a mold cavity plate 106. The nozzle 102 and manifold 104 are arranged such that melt flows from a manifold channel 108, through a nozzle channel 110 and into a mold cavity 112. The nozzle 102 contacts a cold mold plate 114 through spacers 116, which are provided between a nozzle head 118 and the mold plate 114, and seals 120, which are provided between a nozzle tip 122 and the mold plate 114. The nozzle 102 loses heat directly to the mold plate 114 through the spacers 116 and the seals 120. The midsection of the nozzle 102 is not in contact with any part of the mold plate 114 and, therefore, does not lose heat as quickly as the nozzle head 118 and the nozzle tip 122.
Another prior art hot runner nozzle 124 is shown in FIG. 2. The hot runner nozzle 124 screws into a manifold (not shown) and is located in a cavity 126 that is provided in a mold plate 128 of an injection molding apparatus. A heat profile of the nozzle 124 in an operating condition is also shown. The heat profile illustrates the temperature at each position along the length of the nozzle 124. As shown, the heat profile includes a temperature spike 130 that is generally located at the midsection of the nozzle 124. This occurs because there is more heat lost at the ends of the nozzle 124 due to contact with the manifold (not shown) and the cold mold cavity plate (not shown). The midsection of the nozzle 124 does not directly contact any part of the injection molding apparatus and therefore does not lose heat as rapidly as the ends of the nozzle 124.
Uneven heat distributions in nozzles are undesirable because with the increased use of more difficult to mold plastic materials, the melt must be maintained within narrower and narrower temperature ranges. If the temperature rises too high, degradation of the melt will result, and if the temperature drops too low, the melt will clog in the system and produce an unacceptable product. Both extremes can necessitate the injection molding apparatus being shut down for a clean out, which can be a very costly procedure due to the loss of production time.
In order to maintain the temperature of the melt flowing through the melt passage within a desired temperature window, it is known to provide less heat in areas where there is less heat loss, such as the midsection of the nozzle. One method of achieving this is to provide a variable pitch spiral channel in which a heating element is wound, as shown in FIGS. 1 and 2. This arrangement is also disclosed in the applicant's U.S. Pat. No. 4,557,685, which issued on Dec. 10, 1985, and which is incorporated in its entirety herein by reference. The pitch of the spiral channel is customized depending on the heating requirements of the particular molding application. While this improves the heat profile of the nozzle for many applications by reducing the magnitude of the temperature spike, it does not overcome the problem. In addition, this solution is relatively costly because nozzles must be custom made for specific applications.
A further disadvantage of an uneven temperature distribution along the length of a nozzle is that the nozzle is subjected to high stress due to the continuous cycling between higher and lower temperatures. This can result in a shorter nozzle life.
It is therefore an object of the present invention to provide a heat dissipation device for a nozzle that obviates or mitigates at least one of the above disadvantages.